drm/amd/display: Add writeback_config to VBA vars

Adding writeback_config enum to vba_vars_st, replacing old flag.
Initialize to dm_normal.

Signed-off-by: Ilya Bakoulin <Ilya.Bakoulin@amd.com>
Reviewed-by: Charlene Liu <Charlene.Liu@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>